This invention relates to an angle sensor. For more detail, the invention relates to an angle sensor using a magnetoelectric conversion element.
Up to now, a rotational angle displacement sensor which uses a Hall effect as a detecting element is known as an angle sensor for detecting a rotational angle (for example, Japanese Patent Application Laid-Open Publications No. 61-75213 and No. 62-291502). The rotational angle displacement sensor of this kind excels in that a rotational angle can be detected in non-contact condition.
FIGS. 21 and 22 show a plan view and a cross sectional view of a prior rotational angle displacement sensor for detecting a rotational angle displacement of a rotational axis. In FIG. 22, a rotational angle displacement sensor 51 includes a rotational axis portion 52 as a rotational member, a yoke 53 and a permanent magnet 54. The rotational axis portion 52 is connected to a detected rotational axis (not shown) in a body and rotates around an axial center (Z axis) with the detected rotational axis.
The yoke 53 is a cylindrical member with a bottom portion which is formed on a top end of the rotational axis portion 52. A central axial line of a cylindrical portion 53a is coincided with the axial center (Z axis) of the rotational axis portion 52. The cylindrical portion 53a rotates around the central axial line (Z axis) in a body with the rotation of the rotational axis portion 52.
The cylindrical permanent magnet 54 is fixed to an inner side surface of the cylindrical portion 53a. A central axial line of the permanent magnet 54 is coincided with the axial center (Z axis) of the rotational axis portion 52. Accordingly, the permanent magnet 54 rotates around the central axial line (Z axis) in a body with the rotation of the rotational axis portion 52.
The cylindrical permanent magnet 54 is magnetized in such a manner that the front side (lower side in FIG. 21) is a N pole and that the rear side (upper side in FIG. 21) is S pole. A magnetic field which forms a magnetic flux heading from the N pole to the S pole is generated in the cylindrical portion of the magnet 54 as shown by broken line arrow. A characteristic curve shown by X mark in FIG. 4 is a diagram which shows the magnetic flux density distribution on a X axis line (right and left direction) in the cylindrical portion of the permanent magnet 54 under the condition shown in FIG. 21. A characteristic curve shown by X mark in FIG. 5 is a diagram which shows the magnetic flux density distribution on a Y axis line (front and rear direction) in the cylindrical portion of the permanent magnet 54 under the condition shown in FIG. 21.
In a space of the cylindrical portion of the permanent magnet 54, a Hall element 55 as a magnetoelectric conversion element is disposed. A center of the Hall element 55 is coincided with the central axial line (Z axis) of the permanent magnet 54 and the Hall element 55 is arranged along the Y axis direction (front and rear direction) under the condition shown in FIG. 21. The direction of magnetism which the Hall element 55 detects is in parallel with the X axis direction in FIG. 21. When the permanent magnet 54 rotates around the central axial line (Z axis), the relative position between the permanent magnet 54 (N, S poles) and the Hall element 55 changes. The Hall element 55 detects this change of the relative position. The Hall element 55 outputs a detect signal corresponding to the variation of the relative position, namely the rotational angle.
Meanwhile, in the rotational angle displacement sensor 51, the variation of the relative position between the Hall element 55 and the permanent magnet 54, so called, an axis deviation generates structurally easily by the measuring error of the rotational axis portion 52 and so on, the mounting error of the Hall element 55 and so on or the temperature change or the wear. This axis deviation makes the detection by the Hall element 55 generate an error and the detecting accuracy deteriorates.
Namely, under the condition shown in FIG. 21, the variation of the magnetic flux density distribution in the cylindrical portion of the permanent magnet 54 increases as the distance relative to the magnetic poles of the permanent magnet 54 decreases on the basis of the central axis (Z axis). Especially, the variation of the magnetic flux density distribution in the Y axis direction shown in FIG. 5 is larger than the variation of the magnetic flux density distribution in the X axis direction shown in FIG. 4. For example, in FIG. 5, when the disposed position of the Hall element 55 is shifted from the center, the variation of the magnetic flux density becomes extremely large amount.
Accordingly, when the relative position between the Hall element 55 and the permanent magnet 54 is changed, the variation of the magnetic flux density becomes large and the large variation appears as a detection error. Therefore, it is desired to reduce the variation of the magnetic flux density in the X axis and the Y axis directions as much as possible. Namely, it is desired to equalize the magnetic flux density. Because, in case of that the magnetic flux density is equalized, even if the axis deviation generates, the variation of the magnetic flux density is small and the detection error can be reduced.
Further, in the cylindrical permanent magnet 54, the variation of the magnetic flux density in the Z axis direction is also large. Accordingly, in case of that the relative position in the Z axis direction also changes, similar problems occur. Therefore, it is desirable that the magnetic flux density distribution is also equalized in the Z axis direction. Then, a permanent magnet which equalizes the magnetic flux density distribution is suggested (for example, Japanese Patent Application Laid-Open Publication No. 10-132506).
FIGS. 18 and 19 shows a cross-sectional view of permanent magnets 56, 57 which equalize the magnetic flux density distribution in the Z axis direction. In the permanent magnet 56, a circular groove 56b which constitutes a magnetic flux density distribution correction portion is formed on an inner circumferential surface 56a. Further, in the permanent magnet 57, circular projecting portions 57b which constitute a magnetic flux density distribution correction portion are formed at both end portions of an inner circumferential surface 57a. FIG. 20 shows characteristic curves of the magnetic flux density distribution on the Z axis. The characteristic curve shown by X mark is a characteristic curve of the permanent magnet 54 which none is given on the inner circumferential surface and the characteristic curve shown by xe2x97xaf mark is a characteristic curve of the permanent magnets 56, 57 which the magnetic flux density distribution correction portion is formed on the inner circumferential surface. Namely, the variation of the magnetic flux density distribution on the Z axis of the permanent magnets 56, 57 which the magnetic flux density distribution correction portion is formed is smaller than that of the permanent magnet 54 which the magnetic flux density distribution correction portion is not formed.
In case of the permanent magnets 56, 57 which equalize the magnetic flux density distribution on the Z axis by the correction of the form, however, since the form of the magnets is specific, high technique is required for manufacturing the magnets and therefore the manufacturing cost is increased. Namely, in case of that the permanent magnets 56, 57 are made of sintered magnet, a pressing die is required for pressing the magnet powder and it is difficult to die-cut. Further, it is very difficult to accurize the measure of the magnet after sintering. Further, in case of that the permanent magnets are made of plastic magnet, the forming die is also required and it is also difficult to die-cut. Therefore, it is necessary to cut the inner circumferential surface after the forming of the cylindrical magnet for manufacturing the permanent magnets 56, 57 and the manufacturing cost is increased.
The present invention is done for overcoming the above problems and an object thereof is to provide an angle sensor which can equalize the magnetic flux distribution while using inexpensive magnets manufactured easily and which can reduce the detection error even if the relative position between the magnetoelectric conversion element and the magnet changes.
The invention provides an angle sensor comprising; a cylindrical magnet fixed to a rotational member and rotating with the rotation of the rotational member; and a magnetoelectric conversion element disposed in magnetic field generated by the magnet and outputting an electric signal corresponding to the magnetic field; wherein a magnetic flux density distribution correction portion is formed on an outer circumferential surface of the magnet.
The invention also provides an angle sensor comprising; a cylindrical magnet fixed to a rotational member and rotating with the rotation of the rotational member; and a magnetoelectric conversion element disposed in magnetic field generated by the magnet and outputting an electric signal corresponding to the magnetic field; wherein a magnetic flux density distribution correction portion is formed on an outer circumferential surface of the magnet.
According to the invention, two magnetic poles of one side are formed by two piece of the magnets and two magnetic poles of the other side are formed by other two piece of the magnets. And, the magnets forming the magnetic poles of one side and the magnets forming the magnetic poles of the other side are separated from each other and are fixed to the inner side surface of the tubular yoke. Thereby, the distribution of the magnetic flux density in the direction that runs at right angle to the rotational central axis of the rotational member in the tubular yoke being adjacent to the magnetoelectric conversion element is uniformed.
According to the invention, since the magnetic flux density distribution correction portion is formed on the outer circumferential surface of the magnet, it is able to uniform the distribution of the magnetic flux density in the direction of the rotational central axis of the rotational member. Further, it is able to die-cut easily at the forming of the cylindrical magnet. Therefore, the manufacturing of the magnet becomes easy and the manufacturing cost can be decreased.